1. Field of the Invention
The present invention relates to a system and method for obtaining real-time channel condition information on operating DSL (Digital Subscriber Loop) systems, specifically such systems providing high-bandwidth service, and reporting that information for analysis. More generally, the technique of the present invention relates to a method for obtaining and collecting a digital representation of any transmitted analog channel that is subjected to analog to digital conversion at a receiver.
2. Description of the Related Art
Recently, high-bandwidth service has become available over digital subscriber loop (DSL) systems utilizing existing telephone circuits to transmit data to and from subscribers on twisted pair copper telephone lines. Such high-bandwidth service is often referred to as digital subscriber line (also ADSL) service. Hereinafter DSL will be used to refer to the high bandwidth service unless otherwise specified.
FIG. 1 is a block diagram showing a typical DSL deployment to deliver high-speed service in conjunction with POTS (Plain Old Telephone Service). For the sake of simplicity, the figure shows the connection between the central office (CO) 10 and one subscriber 20. Similar connections must be maintained for each subscriber supplied with DSL service.
As shown in the figure, the CO 10 is connected to the Public Switched Telephone Network (PSTN) 40 and also to providers 30 of Internet and Video information. At the CO 10, DSL modem 60 is connected to combiner/splitter 55. Combiner/splitter 55 multiplexes the signal from the modem with the POTS signal from the telephone switch 65, itself supplied by PSTN 40, to supply, over twisted pair line 50, combined downstream POTS and high speed digital service to subscriber 20. The downstream data is received at the subscriber and split, by filtering of the incoming signal, by combiner/splitter 70 at the subscriber into the high-frequency component, which is routed to the subscriber DSL modem 75, and the voice-band (DC-4 kHz) POTS component, which is routed to the subscriber""s telephone 80. Upstream data (that is, data from the subscriber to the CO 10) proceeds from the subscriber""s computer and telephone in the same manner but in the opposite direction.
FIG. 2 is a block diagram illustrating in somewhat more detail the components of the typical DSL system shown in FIG. 1. Broad-band data content, such as Internet and video, to be transmitted downstream to the subscriber is first supplied to digital modulator 5, a component of the DSL modem located at the service provider, in this case the central office. Today, broad-band data is typically coded as multibit words and modulated onto digital representations of the carrier or carriers in the digital domain, depending on the type of modulation. The signal is then passed through a digital to analog (D/A) converter generating an analog signal for transmission over twisted pair 4. The signal generated is then combined, using combiner/splitter 1, with the telephone signal to produce the complete downstream signal.
At the subscriber side, the received downstream signal is divided by combiner/splitter 1, also known as APOTS splitter, typically by the use of high pass and low pass filters, into the high frequency portion of the signal, containing the broad-band content, and the POTS signal, operating in the voice band. After being split off from the POTS signal and appropriate analog signaling, the downstream broad-band content is routed to an analog to digital (A/D) converter 3. The output of the A/D converter 3 constitutes a digital representation of the broad-band portion of the line signal. The signal output from the A/D converter 3 is then fed to digital demodulator 6, which recovers the coded information bits. Upstream data is transmitted by the subscriber and received by the service provider in much the same manner as the downstream data discussed above, but in the opposite direction.
In twisted pair copper loop networks, such as those illustrated in FIGS. 1 and 2, signals are transmitted in differential mode and any Radio Frequency Interference (RFI) normally will be picked up by both wires of the twisted pair approximately equally with the desired signal being determined by the signal between the wires at the receiver. However, even with such inherent interference-canceling attributes, differential lines still may be subject to occasional electrical interference which can interrupt or degrade service. Such interference is often intermittent and may be caused by licensed services such as Amateur Radio or the Military, or by unlicensed services such as power lines or industrial equipment.
Identification of interference on POTS over twisted pair lines has traditionally been accomplished by manual means or by automated methods which make measurements in the telephone voice frequency band (DC to less than 4000 Hz, and typically 300-3500 Hz). Since this is within the range of human hearing, repair personnel and subscribers can often hear the interfering signals on their telephones.
However, the digital modulation techniques that have been developed to provide high bandwidth DSL service on POTS lines are severely affected by the presence of interference and such interference is much harder to detect in DSL transmissions, as compared to voice-band telephone service, since DSL operates in frequency bands above the audible range. As a result, measurements must be made at higher frequencies, above the traditional telephone frequency band, requiring special equipment.
Further, when interference is intermittent in nature a continuous recording scheme must be used to ensure capture of the event. Such approaches require specially trained repair personnel to visit the customer premises, a practice that is time consuming and expensive.
Some currently available DSL modems, such as the Orckit ORvision modem, purport to utilize a built-in Fast Fourier Transform (FFT) spectrum analyzer that operates continually to measure the signal to noise (S/N) ratio, and report and record these measurements. However, this information is limited to information regarding the demodulated signal, and while access to this information can be indicative of the fact that something is wrong, it does not indicate what it is that is wrong. It would be much more helpful to assist in diagnosing a problem with the line to have access to the digitized line signal, post A/D, but pre-demodulation, that is, a digital representation of the actual voltage conditions on the line itself.
Thus, there exists a need for a technique that allows for continuous remote monitoring of transmitted signals without affecting the normal flow of data and without requiring costly visits by human technicians. There also exists a need for reporting the results of the monitoring without affecting transmission in either the upstream or downstream direction.
In consideration of the above problems, in accordance with one advantageous aspect of the present invention, a method is provided for monitoring transmission signal interference in a bi-directional transmission/reception system in which a modulated signal received at a receiver location is subjected at the receiver location to analog to digital (A/D) conversion by an A/D converter, an output of which is routed to an input of a digital demodulator for demodulation. The method comprises: accessing the output of the A/D converter before the output is subjected to demodulation by the demodulator; and storing the accessed data in a storage buffer. The data stored in the storage buffer is available for inspection to assist in determining the presence of signal interference.
In accordance with another aspect of the present invention, there is provided a system operable to receive a modulated downstream signal and transmit a modulated upstream signal. The system comprises: a receiving subsystem comprising an analog to digital (A/D) converter that digitizes the received downstream signal, and a demodulator that demodulates the digitized downstream signal; a transmitting subsystem comprising a modulator that modulates upstream data for transmission in the upstream signal and a digital to analog (D/A) converter that D/A converts the modulated upstream data to produce an analog upstream signal; and a channel monitoring and reporting subsystem. The channel monitoring and reporting subsystem comprises: means for accessing the output of the A/D converter; a memory buffer that temporarily stores samples of the accessed output of the A/D converter; and a combining circuit, responsive to receipt of a command or the expiration of a predetermined time interval, that combines data corresponding to the stored samples with the upstream channel data before modulation for transmission.
In accordance with another aspect of the present invention, there is provided a bi-directional communication system for communication between a service provider and a subscriber, the system comprising: (a) a subscriber system operable to receive a modulated downstream signal from the service provider and transmit a modulated upstream signal to the service provider; and a provider system. The subscriber system comprises: a subscriber receiving subsystem comprising a subscriber analog to digital (A/D) converter that digitizes the received downstream signal, and a subscriber demodulator that demodulates the digitized downstream signal; a subscriber transmitting subsystem comprising a subscriber modulator that modulates upstream data for transmission in the upstream signal and a subscriber digital to analog (D/A) converter that D/A converts the modulated upstream data to produce an analog upstream signal; and a subscriber channel monitoring and reporting subsystem comprising: means for accessing the output of the subscriber A/D converter; a memory buffer that temporarily stores samples of the accessed output of the subscriber A/D converter; and a combining circuit, responsive to receipt of a command or the expiration of a predetermined time interval, that combines data corresponding to the stored samples with the upstream channel data before modulation for transmission. The provider system comprises: a provider receiving subsystem comprising a provider analog to digital (A/D) converter that digitizes a received upstream signal from the subscriber, and a provider demodulator that and demodulates the digitized downstream signal; a provider transmitting subsystem comprising a provider modulator that modulates downstream data for transmission in the downstream signal and a provider digital to analog (D/A) converter that D/A converts the modulated downstream data to produce an analog downstream signal.
In accordance with yet another aspect of the present invention, there is provided an apparatus for receiving and transmitting modulated signals over a transmission medium and monitoring transmission signal interference occurring over the medium. The apparatus comprises: an analog to digital (A/D) converter for converting incoming modulated signals and outputting a digital representation of the modulated signals; demodulation means for inputting the digital representation of the modulated signals and demodulating the input signals; accessing means for accessing the output of the A/D converter before the output is subjected to demodulation by the demodulation means; and storing means for storing the accessed data in a storage buffer, the data stored in the storage buffer being available for inspection to assist in determining the presence of signal interference.
In accordance with still another aspect of the present invention, there is provided a computer-readable medium storing code for causing a processor-controlled apparatus to perform a method for monitoring transmission signal interference in a bi-directional transmission/reception system in which a modulated signal received at a receiver location is subjected at the receiver location to analog to digital (A/D) conversion by an A/D converter, an output of which is routed to an input of a digital demodulator for demodulation. The method comprises: accessing the output of the A/D converter before the output is subjected to demodulation by the demodulator; and storing the accessed data in a storage buffer, the data stored in the storage buffer being available for inspection to assist in determining the presence of signal interference.